1. Field of the Invention
The present invention relates to an organic electrolytic solution and a lithium battery employing the same, and more particularly, to an organic electrolytic solution providing a lithium metal stabilizing effect and improving the conductivity of lithium ions, and a lithium battery employing the same.
2. Description of the Related Art
With the rapid advance of compact portable electronic devices, there is an increasing need for batteries having high energy densities for miniature portable electronic devices.
Lithium sulfur batteries are known as the most promising types of batteries that capable of satisfying the above requirement over other batteries developed by far due to their high energy density. Lithium and sulfur (S8) used as active materials in the manufacture of lithium sulfur batteries have an energy density of about 3,830 mAh/g and 1,675 mAh/g, respectively, and are known as being economical and environmentally friendly. However, there has been no successful commercial use of these active materials in battery systems. The reason why it has been difficult to commercialize lithium sulfur batteries lies in the low availability of sulfur as an active material in electrochemical oxidation reactions, which finally leads to low battery capacitance. In addition, the lifespan of batteries can be shortened due to the outflow of sulfur to electrolyte during oxidation and reduction reactions. If an unsuitable electrolyte is used, sulfur is reduced and separated as lithium sulfide (Li2S) that is no longer available in electrochemical reactions.
To resolve these problems, many attempts have been made to optimize the composition of the electrolyte. As an example, U.S. Pat. No. 6,030,720 discloses use of a mixture of a main solvent such as tetraglyme and a donor solvent having 15 or greater donor number, such as n,n-diethylacetylamide, as an organic solvent of an organic electrolyte.
U.S. Pat. No. 5,961,672 discloses use of an organic electrolytic solution of 1 M LiSO3CF3 in a mixed solvent of 1,3-dioxolane, diglyme, sulfolane, and diethoxyethane for improved lifespan and safety measures, wherein a lithium metal anode is coated with a polymeric film.
When a lithium metal electrode is used as an anode of a lithium secondary battery, the lifespan, capacitance, and other properties of the battery degrade compared with a case using a carbonaceous or graphite electrode. In particular, as a result of repeated charging/discharging cycles, dendrites are separated and grow on the surface of the lithium metal anode, and contact the surface of a cathode, thereby causing shorting out. In addition, the lithium metal corrodes as a result of a reaction with an electrolytic solution at the surface of the lithium anode.
As a solution to these problems, a method of forming a protecting layer on the surface of the lithium metal electrode has been suggested (U.S. Pat. Nos. 6,017,651, 6,025,094, and 5,961,672). To be effective, the protecting layer formed on the surface of the lithium electrode should allow lithium ions to pass through itself as well as act as a barrier to prevent an electrolytic solution from contacting the lithium metal of the anode.
In general, this lithium-protecting layer is formed by the reaction between lithium and a protective layer-forming additive contained in the electrolytic solution after the assembly of the battery. However, the protecting layer formed by this method has poor density, so that a considerable amount of electrolytic solution permeates through cracks present in the protective layer and undesirably react with lithium metal.
Another method for forming the lithium-protecting layer involves processing the surface of the lithium electrode with nitrogen plasma to form a lithium nitride (Li3N) layer on the electrode. However, the lithium nitride layer formed by this method includes grain boundaries through which the electrolytic solution easily permeates, is highly likely to decompose when in contact with water, and has a low potential window. Therefore, the lithium nitride layer is impractical to use.
U.S. Pat. No. 5,272,022 discloses a non-aqueous electrolyte secondary battery employing an anode formed of a carbonaceous active material, in which a mixture of a non-aqueous solvent selected from the group consisting of an acyclic carbonate material, a cyclic carbonate material, γ-butyrolactone, and acetonirile, and a second non-aqueous solvent, such as 1,2-diethoxyethane and 1,2-dimethoxyethane are used for an electrolytic solution.
The non-aqueous electrolyte secondary battery provides enhanced cycling properties when used together with an anode containing a carbonaceous material. However, the non-aqueous electrolyte secondary battery still needs improvements, in terms of electrode stability, when used with a lithium metal electrode.